Epimetheus and Janus may have been part of the same body at one time. They share nearly the same orbit, but unlike co-orbital moons like Tethys, Telesto, and Calypso, their orbit is not stable. In fact, about once every four years they switch places, one being slightly higher than the other. Epimetheus is heavily cratered so if it and Janus were truly once a single entity, the event causing the split must have happened early in its history, billions of years ago. It is irregularly shaped, with ridges and grooves all over its surface, but not as irregular as Atlas or Calypso.